The present invention relates to a method and apparatus for controlling fuel injectors.
Fuel injectors are used to assist in the injection of fuel during the operation of a diesel engine. A fuel injector includes two coils: an open coil and a close coil. To inject fuel into the cylinder, it is necessary to first activate the open coil and then the close coil.
In present designs, each coil includes a high side gate and a low side gate. The injector current is monitored by a low side shunt to ground. The high side gate, connected to the supply voltage, switches off when the injector coil current reaches a desired value. Inductive energy stored in the coil is dissipated by a diode to ground. The low side shunt monitors this decaying current and when it reaches a preset level, the high side gate is turned back on and the coil current starts rising again. The measurement of the current on the low side and the control of it at the high side require level shifting of either the inputs to the drivers or the sensor signals. Also, for applications requiring overlap between the activation of the open and close coils on the same cylinder, measuring on the low side and chopping on the high side results in a system (for an eight cylinder engine) that requires a minimum of eight high side gates and will not allow the use of three wire injectors (where the open and close coils share a lead).
The fuel injector control circuit of the present invention eliminates the necessity of controlling the current on one side and measuring it on the other side. In the present invention, the current to the coil is increased while being monitored by the low side shunt circuitry. The current continues to increase until it reaches the predetermined threshold value. When the predetermined threshold value is reached, the low side switch is switched off and the current begins to decay. Rather than measuring the current during this decay, the low side gate is switched off for a predetermined period of time. When the predetermined period of time elapses, the low side gate is switched back on, causing the current to rise again toward the predetermined value.
Since the falling current is not measured, only timed, the current at the end of the timed cycle may be higher or lower than desired. This is compensated by the rising portion of the cycle where the current is measured. For example, if the delay was too long, and a current dropped too low, the rising current would be on longer, bringing it back up. Likewise, if the delay is too short, causing the current to drop too little, the rising current will be on less, bringing it back to the predetermined value.